A weakly nonlinear baroclinic life cycle is examined with a spherical,
multilevel, primitive equation model. The structure of the initial zonal jet is
chosen so that the disturbance grows very slowly, that is, linear growth rate
less than 0.1 per day, and the life cycles of the disturbance are characterized
by baroclinic growth and followed by barotropic decay. It is found that if the
disturbance grows sufficiently slowly, the decay is baroclinic. As a result,
the procedure for determining this weakly nonlinear jet is rather delicate.

The evolution of the disturbance is examined with Eliassen-Palm flux diagrams,
which illustrate that the disturbance is bounded at all times by its critical
surface in the model's middle and upper troposphere. The disturbance undergoes
two large baroclinic growth/barotropic decay life cycles, after which it decays
by horizontal diffusion. At the end of the first cycle, the zonally averaged
zonal flow is linearly stable, suggesting that the disturbance growth during
the second cycle may have arisen through nonmodal instability. This
stabilization of the disturbance is due to an increase in the horizontal shear
of the zonal wind, that is, the barotropic governor mechanism. It is argued
that this stabilization is due to the large number of model levels.

A quasigeostrophic refractive index is used to interpret the result that as the
linear growth rate of the disturbance is lowered, the ratio of equatorward to
poleward wave activity propagation decreases. A parameter is defined as the
ratio of the horizontal zonal wind shear to the Eady growth rate. It is found
that the growing disturbance tends to be confined to regions of local minima of
this parameter.